The invention relates to an electronic flash unit with an automatic exposure control, which comprises a switching thyristor, which is in series with the flash tube and which can be blocked by the discharge current of a quenching capacitor, which is charged via a resistor and which is paralleled, in series with a further thyristor, to the switching thyristor.
In a known flash unit of this kind, the quenching capacitor is connected, on the one hand, via a resistor, to the positive pole of the main storage capacitor and, on the other hand, to the anode of the switching thyristor. A resistor is connected in parallel with the switching thyristor. The connecting point between the quenching capacitor and the resistor is connected to the cathode of the switching thyristor via a further thyristor, which is triggered by an exposure metering device. As soon as the quantity of light, radiated by the flash tube has reached a desired value, the exposure metering device emits an impulse to the control electrode of the further thyristor, so that the latter is triggered. The quenching capacitor is discharged via the triggered or conductive further thyristor, and the switching thyristor, whereby there is applied to the anode/cathode path of the switching thyristor, for the duration of the discharge, a negative voltage, under the action of which the charge carriers are withdrawn from the barrier or blocking layer of the switching thyristor and the switching thyristor blocks or becomes non-conductive. The resistor which connects the quenching capacitor to the positive pole of the storage capacitor is proportioned in such a way that after the decay of the discharge current from the quenching capacitor the residual current, which flows from the positive pole of the storage capacitor via the resistor and from the opened further thyristor, sinks below the holding current of the further thyristor and the latter thereby blocks. Now the quenching capacitor can be recharged to the operating voltage via the resistor connecting it to the positive pole of the main storage capacitor and in the case of a renewed ignition of the flash tube the quenching circuit for the switching thyristor is ready for operation.
If one wants to generate a rapid sequence of short light impulses with this flash unit, then a minimum time interval between the single impulses has to be observed, the greatest proportion of which interval being determined by the charging time of the quenching capacitor. The charge time constant of the quenching capacitor, which is dependent on the capacitance value of the quenching capacitor and the resistance value of the resistor which is connected between the quenching capacitor and the positive pole of the main storage capacitor, is not to be shortened at will, since the capacitance value of the quenching capacitor has to be proportioned to the recovery time or circuit commutated turn-off time of the switching thyristor and the resistor has to be proportioned to the magnitude of the holding current of the further thyristor. The value of resistance must therefore have a specific minimum magnitude, so that the quotient from the operating voltage or the voltage applied to the storage capacitor respectively and the value of resistance is smaller than the holding current of the further thyristor. Only in this case is it insured that the further thyristor can be returned to its blocking condition. Since the value of the holding current of a thyristor is very small, the resistor between the quenching capacitor or the anode of the further thyristor respectively and the positive pole of the main storage capacitor has to be proportioned very large with a conventional operating voltage of the flash unit of 360 volts, whereby the charging time of the quenching capacitor is increased which, in turn, only allows an impulse sequence with a great time interval.
It is therefore an object of the present invention to provide an electronic flash unit of the kind mentioned above, in which the time span from the emission of a quenching signal to the further thyristor by the exposure metering device until the renewed operating readiness of the quenching circuit for the switching thyristor is substantially shortened compared to the known flash units.